With the miniaturization of cameras and increasing pixel counts, significant advances are being observed in cell miniaturization of downsizing pixel cells while increasing the number of electrons handled per unit area in CCD solid state image sensing elements used for HD video cameras and digital still cameras in classes over 10 million pixels for consumer and business applications. In addition, imaging with moving pictures is an indispensable function for these CCDs and this is required to be achieved concurrently with the speed-up of horizontal CCD drive.
Since the transfer length of a repetition section of a horizontal CCD is shortened as cells become finer, transfer deterioration is less likely to be a problem. The number of saturated electrons in the repetition section can also be secured by only widening the repetition section, and is thus unlikely to be a problem either. However, the horizontal CCD outlet that makes a connection from the repetition section of the horizontal CCD to a floating diffusion (FD) section that performs signal electron-voltage conversion is shaped such that the horizontal CCD of the repetition section is gradually narrowed down to collect signal electrons from the repetition section of the horizontal CCD having a large design size to the FD section having a small design size. Therefore, the gate length of the outlet and the width of the CCD cannot be reduced and the number of saturated electrons at the horizontal CCD outlet cannot be maintained, and it is therefore difficult to improve transfers at the outlet for the purpose of increasing the speed of horizontal CCD drive.
Furthermore, in order to downsize pixel cells, which requires an increase of the number of electrons handled per unit area, it is effective to make n- and p-type diffusion layers forming a CCD channel shallower and have a higher concentration.
However, since this technique forms a CCD channel so as to have a higher concentration and shallower structure, potential variations occur only locally during a transfer, the transfer electric field in the entire channel attenuates and transfer deterioration at the horizontal CCD outlet becomes more noticeable. For this reason, with micro cells of 2.0 μm or less in a CCD solid state image sensing element, characteristics such as the number of saturated electrons and horizontal transfer efficiency at the horizontal outlet become dominant and it is difficult to realize high-speed horizontal driving while maintaining a high number of saturated electrons. Especially, it is difficult to realize high-speed driving.
Furthermore, the horizontal CCD outlet is narrowed down so as to become gradually narrower than the width of the horizontal CCD at the repetition section to collect electrons at the FD section that follows the horizontal CCD outlet. However, since the number of saturated electrons also needs to be satisfied simultaneously, the horizontal CCD is drastically narrowed down at an end of an offset gate electrode which is the end of the horizontal outlet and at an end of the FD section. For this reason, this structure is liable to cause deterioration of a transfer from right below the offset gate electrode which is the end of the horizontal outlet to the FD section.
Therefore, in order to further proceed with high-speed horizontal driving by micro cells, a new well structure or layout needs to be introduced to the horizontal CCD outlet.
A conventional solid state image sensor designed for high-speed driving and improvement of horizontal transfer efficiency will be explained using FIG. 18 and FIG. 19.
FIG. 18 shows a schematic plan view of the horizontal CCD outlet of the solid state image sensor in a conventional example and FIG. 19 shows a cross-sectional view in a direction parallel to the charge transfer direction in the conventional example.
In FIG. 18, a region 302 including a gate electrode 312, gate electrode 313, gate electrode 314, gate electrode 315, gate electrode 316, gate electrode 317 and gate electrode 318 formed above an n-type region 310, p-type region 308 and p-type region 309 is the horizontal CCD outlet that sends signal electrons from a horizontal CCD to a voltage conversion section. A region 304 including the n-type region 310, an n-type region 324 and a gate electrode 319 formed above the n-type region 310 is a reset drain section that discharges signal electrons from the voltage conversion section. A region 303 interposed between the region 302 and region 304 is an FD section made up of the n-type region 310, p-type region 308, p-type region 309, contact 325 and AL wiring 327, for converting signal electrons to a voltage. The n-type region 310, which is the horizontal CCD in the region 302 being the horizontal outlet, is gradually narrowed down toward the region 303 being the FD section and drastically narrowed down to the width of the n-type region 310 in the region 303 being the FD section at an end adjoining the region 303 of the gate electrode 318.
The structure will further be explained using FIG. 19 which shows a cross section in a direction 301 parallel to the charge transfer direction including the gate electrode 312, gate electrode 313, gate electrode 314, gate electrode 315, gate electrode 316, gate electrode 317, gate electrode 318, region 303, gate electrode 319 and n-type region 324.
Furthermore, in FIG. 19, a p-type well 307 is formed in the depth of a semiconductor substrate 306. The p-type region 308 is formed in contact with the p-type well 307 on the surface side of the substrate 306. The p-type region 309 is formed in contact with and on the p-type region 308 on the surface side of the substrate 306. The n-type region 310 is formed in contact with the p-type region 309 on the surface of the substrate 306. The n-type region 310 and p-type region 309 are formed with a high concentration and shallowly to increase the number of electrons handled by pixel cells. A gate insulating film 311 is formed on the surface of the substrate 306 of the n-type region 310. The gate electrode 313, gate electrode 315 and gate electrode 317 are formed respectively through the gate insulating film 311. These gate electrodes are storage gates for accumulating signal electrons during a transfer by the horizontal CCD. Since the n-type region 310 and p-type region 309 of the region 302 which is the horizontal CCD outlet are shaped like a trapezoid narrowed down toward the region 303, the gate electrodes closer to the region 303 have a greater electrode length to secure the number of saturated electrons right below the gate electrode 313, gate electrode 315 and gate electrode 317 which are the respective storage gates of the region 302, which is the horizontal outlet.
There is a relationship as follows: gate length in horizontal repetition section<gate length of electrode 313<gate length of electrode 315<gate length of electrode 317. The gate electrode 314 is formed on the gate insulating film 311 so as to adjoin the gate electrode 313 and the gate electrode 315 through an insulating film 326. The gate electrode 316 is formed on the gate insulating film 311 so as to adjoin the gate electrode 315 and the gate electrode 317 through the insulating film 326. The gate electrode 318 is formed on the gate insulating film 311 so as to adjoin an end of the gate electrode 317 not adjoining the gate electrode 316 through the insulating film 326. The gate electrode 312 is formed on the gate insulating film 311 so as to adjoin an end of gate electrode 313 not adjoining the gate electrode 314 through the insulating film 326. In the n-type region 310, there are formed a p-type region 320 right below the gate electrode 312, a p-type region 321 right below the gate electrode 314, a p-type region 322 right below the gate electrode 316 and a p-type region 323 right below the gate electrode 318, respectively on the surface of the substrate 306. When transferring a signal using two-phase driving in the horizontal CCD, the p-type region 320, p-type region 321, p-type region 322 and p-type region 323 function as barriers to restrain backflow of signal electrons and promote the transfer. The horizontal CCD is made up of the n-type region 310, p-type region 309, p-type region 320, p-type region 321, p-type region 322 and p-type region 323. The gate electrode 319 to control discharge of signal electrons is formed next to the gate electrode 318. The n-type region 324 is formed in contact with the n-type region 310 right below the gate electrode 319. The region 304 including the gate electrode 319 and n-type region 324 is the reset drain section. The AL wiring 327 is connected to the contact 325 formed on the surface of the substrate 306 of the n-type region 310, penetrating through the insulating film 326 and gate insulating film 311 between an end of the gate electrode 318 not adjoining the gate electrode 317 and an end of the gate electrode 319 not adjoining the n-type region 324. The region 303 between the end of the gate electrode 318 not adjoining the gate electrode 317 and the end of the gate electrode 319 not adjoining the n-type region 324 is the FD section that performs signal electron-voltage conversion.